DE 22 33 700 C2 provides hard metal substrate bodies consisting of a mixture of at least one metal serving as a bonding agent and at least one metal carbide with high hardness with a coating of aluminum oxide or zirconium oxide. The substrate body can consist, particularly of tungsten, titanium, tantalum or niobium carbide or of a composite carbide of tantalum and niobium, whereby for instance the metals cobalt, iron or nickel serve as bonding agent. In the literature hard metals based on titanium carbide or titanium carbonitride are frequently called cermets, which should also be understood as substrate materials which are combinations of a hard metal with a ceramic, i.e. nonmetallic components. According to DE 22 33 700 C2, the mentioned layers of .alpha.-aluminum oxide are applied by using a CVD process at a substrate temperature of 1000.degree. C.
Correspondingly, the same is valid for hard metal bodies. DE 22 53 745 A1 describes a sintered hard metal substrate body, an intermediate layer of titanium carbide and an outer layer of aluminum oxide, whereby the titanium carbide intermediate layer is supposed to be applied at 1000.degree. C. and the second aluminum oxide layer at 1100.degree. C. by the CVD process. As stated particularly in DE 28 25 009, column 2, lines 28 and following, hard, polycrystalline and compact .alpha.-aluminum oxide layers should normally be produced only at deposition temperatures above 950.degree. C. According to the state of the art, at lower deposition temperatures loose and powdery deposits are formed, consisting mostly of the .gamma.-modification and/or the .DELTA.-modification of aluminum oxide. However, at deposition temperatures of approximately 1000.degree. C. and above the aluminum oxide phase is normally the .gamma.-modification, considered suitable for the coating of tools. In order to avoid the danger of multiphase aluminum oxide coatings, which supposedly occur at deposition temperatures below 1000.degree. C. and which show a considerable mechanical weakness causing premature tool failure, it is proposed that the aluminum oxide coating should consist of at least 85% of the .gamma.-modification and that optionally a balance consisting of the .alpha.-modification form zones or spots of a magnitude of maximum 10.mu.m on the surface. For the deposition the CVD process at temperature of approximately 1000.degree. C. is proposed.
In order to avoid the problems arising at high deposition temperatures, the German Patent Document DE 32 34 943 describes the deposition of an amorphous aluminum oxide layer. However, thorough tests performed with amorphous aluminum oxide layers deposited by the PVD process have shown that purely amorphous aluminum oxide layers have a glass-like breaking behavior and therefore cannot yield any significant improvements of the wear factor. In interrupted cutting operations these layers have a tendency to crack.
The German Paten Document DE 24 28 530 A1 proposes a method for the protection against corrosion and wear of a metal part which contains at least one element of the Group I B of the periodic table of elements in pure or alloyed state, and wherein on the surface of this part a layer of amorphous and transparent aluminum oxide is applied through chemical deposition from the vapor phase. However, amorphous layers applied at temperatures between 300.degree. and 800.degree. C. are far less stable against thermal influences than for instance the modification of the aluminum oxide (.alpha.-Al.sub.2 O.sub.3) known as corundum.
It is basically also known to use aluminum oxide layers as protective coatings against hot gas corrosion, e.g. in internal combustion engines. In this case special requirements have to be met not only relating to the mechanical stability of the coating, but also relating to the sealing characteristics of the coating. Layers consisting totally or partially of .gamma.-Al.sub.2 O.sub.3 undergo at high temperatures a conversion into .alpha.-Al.sub.2 O.sub.3, whereby fissures occur. Therefore they are not suited as protective layers against hot gas corrosion. According to the state of the art, this can be achieved only by comparatively thick (approximately 500 .mu.m) ceramic layers applied by thermal spraying.